Media access control of a point-to-multipoint link in a packet switched network is a mechanism for determining which of a plurality of nodes at the multipoint end of the link is allowed to send a packet over the shared medium to a single central node located at the other end of the link. In a conventional media access control mechanism utilised in two-way TDMA transfer links, the central node appends permit information, commonly called a permit, to every packet that it sends. The permit is addressed to one of the multipoint nodes and specifies which time slot in the future packet stream can be utilised by the addressed multipoint node to send a packet.
Permits are allocated in response to requests. The multipoint nodes send requests, typically in the form of queue size information, when packets are present at the node awaiting transfer. The central node sends corresponding permits and will also manage the allocation of priorities when the requests for permits arrive more frequently than permits can be sent. To this end, each multipoint node has a queue that stores packets until the node is permitted to pass them on.
Each multipoint node records the number of queued packets awaiting transmission so that a corresponding permit request may be sent to the central node. The central node also holds a record of the queue lengths at each multipoint node. On the basis of this queue record, the central node issues permits to those multipoint nodes requiring channel capacity.
The queue record in the central node is updated by queue information in the form of a permit request received from the multipoint node. This request represents the current queue size at the multipoint node and is used to overwrite the recorded queue size at the central node. The recorded queue size at the central node is also decremented each time a permit to send a packet is transmitted to the corresponding multipoint node.
The transfer of information between a central node and a multipoint node involves a delay, which depends on the available bandwidth and length of the transfer medium. A delay also occurs at the multipoint node when processing permits received from the central node. Hence there exists a window during which a multipoint node may transmit a permit request and the central node may transmit permits to the same multipoint node on the basis of the queue record before receiving the permit request. When the permit request reaches the central node, the queue record will be overwritten by an erroneous queue size value. While this error will not cause the loss of packets, it can lead to more permits being sent than are actually required. It is possible that the discrepancy between the actual and recorded queue size will be mitigated to some extent by further packets arriving at the multipoint node, however, on balance there is a high probability that the central node will issue permits that cannot be used. Channel capacity will thus be wasted.